Oxide layers, in particular ceramic and especially aluminum oxide (Al2O3), are used as coating material for a multiplicity of applications which impose stringent demands in respect of heat stability and heat shock stability or resistance to wear, oxidation or hot corrosion, thermal stability and electrical insulation.
Such layers can act as a diffusion barrier for ions and have high chemical stability and radiation resistance. They are therefore used in many fields. Thus, by way of example, aluminum oxide serves as an insulation material in the field of microelectronics. Owing to its chemical resistance and biocompatibility, it is also used in the field of medicine. Coatings comprising oxides are a good option for protecting surfaces against oxidation or hot corrosion, for example. This high chemical stability coupled with highly advantageous mechanical properties make oxides an ideal material for protective layers.
In this case, the production of suitable oxide compounds constitutes a major challenge; particularly the production of suitable oxide compounds having high homogeneity and purity is difficult. Thus, by way of example, aluminum oxide is present either as an amorphous phase or in various crystalline modifications with different properties. Said modifications have the more advantageous properties for protective coatings, since amorphous phases are normally softer. Crystalline aluminum oxide can be present in various modifications, of which only α-Al2O3 (corundum) is thermodynamically stable. The others, so-called transition aluminum oxides, such as γ, δ, η, θ, χ, χ′-Al2O3 and Al2O3-KII, are metastable and can be irreversibly converted into α-Al2O3. Above 1200° C., corundum is the only stable modification. In this case, corundum is also the hardest modification of aluminum oxide. The low ionic conductivity and its high thermodynamic stability make it an important coating against oxidations.
The prior art discloses a variety of methods for producing coatings and films composed of aluminum oxide, such as, for example, chemical vapor deposition (CVD), physical vapor deposition (PVD), hydrothermal synthesis, sputtering or the sol-gel method.
U.S. Pat. No. 6,521,203 describes the production of α-Al2O3 by calcination of aluminum hydroxide, produced by hydrolysis of aluminum isopropoxide, at a temperature of 700 to 1300° C. However, this method does not permit the production of thin coatings.
U.S. Pat. No. 5,302,368 describes the production of coatings by applying a dispersion of aluminum hydroxide and/or a transition aluminum oxide in aqueous medium. After adjusting the slurry and spray drying, the dry powder is calcined in the presence of a chlorine-containing substance at 1100° C. to 1500° C.
For industrial applications for depositing oxide layers, chemical vapor deposition (CVD) at high temperatures, normally around 1000° C., is normally used since this technique affords the possibility of coating even complex geometries in conjunction with well-controllable thickness of the coating.
U.S. Pat. No. 5,654,035 describes such a process wherein the body to be coated is brought into contact at high temperature with a hydrogen carrier gas and a hydrolyzing or oxidizing agent, said hydrogen carrier gas containing one or more aluminum halides. In addition, U.S. Pat. No. 6,713,172 describes the application of this method for coating cutting tools, once again at high temperatures of approximately 1000° C.
U.S. Pat. No. 7,238,420 describes a nanotemplate composed of relatively pure and fully crystalline α-Al2O3 on a metal alloy. In the production method disclosed, crystalline α-Al2O3 is produced with the aid of CVD directly on the surface of the alloy. For this purpose, the latter is pretreated before the deposition with a CO2/H2 mixture at high temperatures of 1000° C. to 1200° C.
All the methods described require high temperatures. The latter not only limit the possible substrates but can also lead to thermal flaws in the coating. Thus, the oxide coatings and the substrate often have different coefficients of thermal expansion of film and substrate, which leads to thermally induced flaws in the coating.
In order to avoid the high temperatures, major efforts have been undertaken to develop methods which make it possible to deposit oxide layers at lower temperatures, for example physical vapor deposition (PVD).
U.S. Pat. No. 5,683,761 describes a method for depositing α-Al2O3 with the aid of electron beam PVD. However, the substrate has to be heated to approximately 1000° C. Therefore, the deposition of pure oxide, in this case α-Al2O3, also requires high temperatures.
Variants of the CVD method, such as plasma assisted/enhanced chemical vapor deposition (PACVD/PECVD) or metal organic chemical vapor deposition (MOCVD) likewise afford the possibility of using lower temperatures.
Thus, Pradhan et al. (Surf. Coat. Tech. 176 (2004) 382-384) describes that the use of metallo-organic aluminum compounds leads to the formation of crystalline aluminum oxide at low temperatures (higher than 550° C.). Only amorphous films were obtained at lower temperatures.
Although MOCVD methods afford many advantages, such as, for example, lower temperatures, simple processes, uniform coatings or the use of a single precursor, they also lead to carbon-like impurities in the coating.
The degree of crystallinity and the crystalline phases within the deposited oxide layer are very important for the mechanical properties thereof. A pure phase having high thermal and mechanical stability is distinctly preferred to a mixture of different phases. However, this necessitates a suitable heat treatment of the coated substrate which leads not only to the transformation into the desired phase but additionally to a densification of the coating, which is likewise of great importance for the mechanical stability of the layer. Such a heat treatment often requires temperatures of greater than 1200° C., which are not suitable for many substrates.
In order to avoid the heating of the entire coated substrate, a local heat treatment is appropriate. In this context, lasers have already been used successfully for the treatment of such ceramic materials (laser sintering). In this case, the coating is heated in a small region with the aid of a laser beam. These methods are used precisely in the field of oxide ceramics since they absorb in the range of the CO2 lasers used. In this case, one particular problem is the formation of thermally induced flaws during the resolidification and cooling of the material. They result from the brittleness of the ceramics and from the high temperature gradient between the region of action and the surrounding material, and also the different coefficients of thermal expansion of coating and substrate.
Thus, Triantafyllids et al. (Appl. Surf, Sci. 186 (2002) 140-144) and WO 2007/102143 describe the occurrence of thermally induced cracks during laser sintering. Such defects naturally influence the density and stability of the coating and the homogeneity of the phase transformation.
These effects can be reduced by adding binders to the oxide compounds, e.g. the aluminum oxide particles. Thus, U.S. Pat. No. 6,048,954 describes such a binder composition for inorganic particles having a high melting point. Although such binders increase the densification of the coating, they can only be employed for pulverulent materials and the binder and also the residues thereof have to be removed after the laser sintering or even remain in the oxide layer.
Since the efficiency of laser sintering is highly dependent on the absorption of the material to be sintered, absorption is an important criterion. In this case, the binder can also contribute to the absorption. Thus, U.S. Pat. No. 6,007,764 describes the use of a mixture of absorbent and ceramic particles in order to improve the absorption. Zheng et al. (Mat. Lett. 60 (2006) 1219-1223) use polystyrene-coated aluminum oxide particles in order to optimize the absorption for CO2 lasers. Said particles exhibited better absorption and therefore also more uniform heating and a reduced temperature gradient. However, organic material always remains as a residue in the coating.
DE 10 2006 013 484 A1 describes the production of an element/element oxide composite material, that is to say a material containing an element and the corresponding element oxide, in this case nanowires comprising a metal core and an oxide sheath.
The disadvantage of most methods for producing oxide layers resides in the high temperatures of the method. In the case of the laser sintering methods, the essential disadvantages are that only very specific lasers in a certain wavelength range, usually CO2 lasers, are suitable for being used; the precursors used do not absorb other wavelengths. This causes high temperature gradients and leads to a higher loading of the substrate and to thermally induced cracks and defects. Therefore, the addition of additional binders is often necessary in order to increase the absorption of the laser energy and to achieve a high quality of the coating. However, residues of said binders remain in the coating. Moreover, the production of high-quality and fault-free coatings requires a high degree of experience since influencing of the underlying substrate or excessive heating has to be avoided.
Problem
The present invention addresses the problem of overcoming the disadvantages of the prior art in the production of oxide layers as a coating composition. The problem addressed by the invention is, in particular, that of specifying a method which makes it possible to produce suitable oxide compounds as a coating composition.